Remove Default impl for ConnectionId (#7452)

We noticed the following message in my logs when trying to debug some
lag when collaborating:

```
2024-02-06T09:42:09-08:00 [ERROR] error handling message. client_id:3, sender_id:Some(PeerId { owner_id: 327, id: 1123430 }), type:GetCompletions, error:no such connection: 0/0
```

That `0/0` looks like a bogus connection id, constructed via a derived
`Default`. We didn't ever find a code path that would *use* a default
`ConnectionId` and lead to this error, but it did seem like an
improvement to not have a `Default` for that type.

Release Notes:

- N/A

Co-authored-by: Marshall <marshall@zed.dev>

crates/collab/src/db/queries/projects.rs

@@ -778,7 +778,7 @@ impl Database {
             let left_project = LeftProject {
                 id: project_id,
                 host_user_id: project.host_user_id,
-                host_connection_id: project.host_connection()?,
+                host_connection_id: Some(project.host_connection()?),
                 connection_ids,
             };
             Ok((room, left_project))